Police are looking for a 7-year-old girl and an 8-year-old boy.
Luna Potts and Hunter Potts are believed to be with Benjamin Martin Moore of Eastend, Sask., according to an Amber Alert issued at the request of Shaunavon RCMP.
The 50-year-old man might go by different names.
The amber alert described Moore as 5 feet, 10 inches tall, 200 pounds and black hair.
CBC News previously reported Moore served a two year and two month sentence for inappropriately touching a young child, possessing child pornography and other offences.
In 2011 Moore was deemed a high-risk to reoffend and his release prompted a warning from police in Regina.
Shaunavon RCMP said the group may be travelling in a 2015 dark blue Chevy Equinox, with the Alberta licence plate CGC2492.
The mother of the children is 45-year-old Leah Potts. She is believed to be travelling with Moore and the children, according to the alert. The group might have entered the United States.
People who see the children, Potts or Moore or have information regarding their whereabouts are asked to call 911 or 1-or 1-877-SOAMBER (762-6237).